A majority of golf balls sold today have covers formed either of balata or a blend comprising one or more ionomers. Balata covered, wound balls are typically preferred by skilled golfers because such balls have a high spin rate, allowing skilled players to draw or fade the ball, and to stop the ball quickly on the green with approach shots. However, balls with balata covers are expensive, and are easily cut or otherwise damaged when miss-hit, e.g., by a less than skilled player.
Therefore, most amateur golfers prefer two-piece golf balls having covers which comprise an ionomer or ionomer blend. Two-piece balls with ionomer covers are less expensive than wound, balata covered balls, and are highly resistant to cutting or other damage. However, these balls have a hard feel and a low spin rate when compared to balata covered balls, so that they are more difficult to control.
Although golf ball manufacturers have attempted to produce golf ball covers that provide the spin rate of balata with the cut resistance of an ionomer, i.e., a copolymer of an .alpha.-olefin and an .alpha.,.beta.-unsaturated carboxylic acid, by forming blends of ionomers that differ in hardness, acid content, and in the metal atom used to neutralize the carboxylic acid groups on the polymer chain, none of these attempts have been fully successful.
U.S. Pat. No. 3,819,768 to Molitor discloses golf ball covers comprising blends of zinc and sodium ionomers, which overcome cold cracking problems.
U.S. Pat. No. 4,526,375 to Nakade discloses covers comprising a single ionomer in which part of the carboxylic acid groups has been neutralized with sodium and another part has been neutralized with magnesium or zinc to overcome a fatigue fracture problem in covers comprising sodium/zinc ionomer blends, as well as a cold cracking problem in covers comprising a single ionomer.
U.S. Pat. No. 5,000,459 to Issac discloses golf ball covers made from a lithium ionomer resin, wherein up to 50 percent of the lithium ionomer resin is replaced with at least one sodium ionomer resin. The resulting golf balls are found to be comparable in durability and initial velocity to golf balls having a cover with lithium ionomer only.
U.S. Pat. No. 4,911,451 to Sullivan et al. discloses a golf ball cover including about 10 to 90 percent by weight of an ethylene-acrylic acid copolymer, containing about 11 percent by weight acrylic acid with about 40 percent of the carboxylic acid groups neutralized with sodium, and about 90 to 10 percent by weight of at least one ethylene-acrylic acid copolymer, containing about 11 to 16 percent acrylic acid with about 10 to 40 percent of the carboxylic acid groups neutralized with zinc. Although not exemplified or even further discussed, ionomers neutralized with metal ions other than zinc and sodium are also disclosed. The exemplified golf balls are said to have the durability of balls having ethylene-methacrylic acid ionomer covers, but with an improved coefficient of restitution.
U.S. Pat. No. 5,492,972 to Stefani discloses a golf ball cover composition of a blend comprising 95 to 50 parts by weight based on 100 parts by weight resin (phr) of a low flexural modulus sodium ionomer resin and about 5 to 50 phr of a lithium ionomer resin. In addition, a second sodium ionomer, having a flexural modulus and melt flow index that are different from that of the first sodium ionomer, can be added to the blend. The spin rate of such a ball is improved over typical two-piece balls, and approaches the spin rate of a wound, balata covered ball. However, the initial velocity of the disclosed golf balls is less than that of two-piece balls having covers comprising a lithium ionomer.
U.S. Pat. No. 4,884,814 to Sullivan discloses golf ball covers comprising blends of a high flexural modulus ("hard") ionomer and a low flexural modulus ("soft") ionomer. The hard ionomers are copolymers of an olefin and an unsaturated monocarboxylic acid, neutralized with sodium or zinc, and have a flexural modulus of about 30,000 to 55,000 p.s.i. and a Shore D hardness of about 60 to 66. The soft ionomers are terpolymers of an olefin, an unsaturated monocarboxylic acid, and an acrylate ester. These terpolymers are neutralized with zinc or sodium, and have a flexural modulus of about 3,000 to 7,000 p.s.i. and a Shore D hardness of about 25 to 40. The resulting cover is disclosed as being intermediate between balata and hard Surlyn in softness to such a degree that adequate spin can be imparted to the ball by a skilled golfer.
U.S. Pat. No. 5,120,791 to Sullivan discloses golf ball cover compositions that provide enhanced carrying distance when compared to the golf balls of U.S. Pat. No. 4,884,814 without sacrificing playability or durability. The disclosed compositions are blends of at least one hard ionomeric resin, preferably an acrylic acid ionomer, and at least one acrylic acid based soft ionomer. Hard ionomers neutralized with sodium, zinc, magnesium, or lithium, having a flexural modulus of 15,000 to 70,000 p.s.i. and a Shore D hardness of greater than 50 are disclosed. However, only zinc and sodium ionomers are exemplified. The disclosed soft ionomers are sodium or zinc neutralized terpolymers of an olefin, acrylic acid, and an unsaturated acrylate ester monomer, and have a flexural modulus of about 2,000 to 10,000 p.s.i. and a Shore D hardness of about 20 to 40. Golf balls having such cover blends reportedly have an improved coefficient of restitution and spin rate when compared to prior art golf balls having covers with hard-soft ionomer blends.
U.S. Pat. No. 5,415,937 to Cadorniga et al. discloses a golf ball cover material of a blend of 80-50 percent of a high stiffness ionomer and 20 to 50 percent of a very low modulus ionomer ("VLMI") to improve the feel and playability of the ball when compared to a standard ionomer cover, while retaining the distance and resilience of the prior art balls. High stiffness ionomers are defined as a copolymer of 75 to 80 percent of an olefin and 20 to 25 percent of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid, having 10 to 90 percent of the acid groups neutralized with sodium, lithium, zinc, or magnesium. The high stiffness ionomers preferably have a melt index of 0.5 to 5.0 g/10 min., a Shore D hardness of at least 70, and a flexural modulus of 60,000 to 120,000 psi. The VLMI are defined as terpolymers, consisting of 67 to 70 percent by weight of ethylene, 20 to 23 percent by weight of n- or iso-butyl-acrylate, and 10 percent by weight of methacrylic acid, where 10 to 90 percent of the acid groups are neutralized by sodium, zinc, or lithium. The VLMI preferably have a melt index of 0.5 to 10 g/10 min, a Shore D hardness of 20 to 50, and a flexural modulus of 2,000 to 8,000. Golf balls having covers incorporating the disclosed blends have a slightly improved coefficient of restitution and initial velocity with spin rates that range from slightly better than prior art blends to significantly lower, depending upon the particular blend and the club used in the test, i.e., driver, 5-iron, or pitching wedge.
U.S. Pat. No. 5,324,783 to Sullivan discloses golf ball covers having an intermediate modulus with improved distance. The covers include a blend of a "relatively low amount", i.e., about 10 to 30 weight percent, of soft ionomer resins and about 90 to 70 weight percent of hard ionomer resins. The soft ionomers are terpolymers of an olefin, acrylic or methacrylic acid, and an unsaturated acrylate ester. Although U.S. Pat. No. 5,324,783 discloses hard and soft ionomers neutralized with sodium, zinc, magnesium, and lithium, only blends of sodium and zinc ionomers are exemplified and discussed. Golf balls incorporating the disclosed cover composition have an improved spin rate compared to balls incorporating only hard ionomers.
British Patent No. GB 2,286,594 to Egashira discloses golf balls having covers formed from a blend of a hard magnesium ionomer, an ethylene-methacrylic acid-unsaturated carboxylate terpolymer, and a hard lithium ionomer. There is no disclosure of blends of ionomers neutralized with a single type of metal ion.
WO 94/240204 to Iwase et al. discloses golf balls having covers formed from a blend of a hard magnesium ionomer, an ethylene-methacrylic acid-isobutyl terpolymer, and a hard lithium ionomer. Preferably, at least one ionomer containing a monovalent cation is blended with a magnesium ionomer. Again, there is no disclosure of blends of ionomers neutralized with a single type of metal ion.
U.S. Pat. No. 5,691,418 and WO 97/02318 to Hagman and Statz, which have identical disclosures, disclose golf balls comprising blends of a soft ionomer terpolymer, i.e., a VLMI, and a hard ionomer, where the ionomer terpolymer and the hard ionomer are both neutralized with lithium or magnesium, where soft lithium ionomers are preferred over a soft magnesium ionomers. The disclosed soft ionomers have a flexural modulus in the range of about 5,000 to about 35,000 psi. Although there is a teaching that soft ionomers having a flexural modulus higher than 35,000 may be produced, there is no disclosure that soft ionomers having a flexural modulus of less than about 5,000 psi are desirable, or are even possible. Moreover, the disclosure states that soft ionomers having a flexural modulus of less than 10,000 psi should be blended with a hard ionomer so that the final flexural modulus is at least 10,000 to 35,000 psi or higher.
U.S. Pat. No. 5,688,869 to Sullivan discloses golf ball cover compositions containing a blend of a metal cation neutralized high acid ionomer resin and a soft ionomer resin containing an acrylate ester class comonomer. Disclosed metal cations include ions of sodium, zinc, manganese, lithium, potassium, magnesium, calcium, and nickel. The disclosed soft ionomers have a hardness of from about 20 to about 40 Shore D and a flexural modulus of from about 1,500 to 15,000 psi. However, specific ionomers having a flexural modulus of 1,500 are not disclosed, and there is no teaching of which metal cations will provide such a low flexural modulus. Cover compositions formed by blending a high acid copolymer of ethylene and acrylic acid with a terpolymer of ethylene, acrylic or methacrylic acid, and an acrylate or methacrylate class ester are also disclosed. The blend of acid polymers is then neutralized with 3 to 4 parts by weight magnesium oxide based on 100 part of the acid polymers to form a blend of magnesium ionomers. However, although the flexural modulus of each of the resulting hard and soft ionomers and that of the resulting blend is not disclosed, the hardness of the resulting cover compositions is at least 93 Shore C.
Therefore, a need exists for golf ball compositions having improved durability and spin characteristics, as well as the initial velocity and carry of two-piece balls having a hard ionomer cover. The present invention provides such golf ball compositions.